Demand for communication traffic has significantly increased with the progress of multimedia networks, and multi-stage optical relay transmission utilizing optical amplifiers whose amplification media are Erbium Doped Fibers (EDFs) have played major roles for marketizing communication systems in a multimedia society.
As one of performance indexes of an optical amplifier performing the multi-stage optical relay transmission, a Noise Figure (NF) may be cited. The NF indicates how much the Optical Signal to Noise Ratio (OSNR) of a light signal on an output side is deteriorated with respect to the OSNR of a light signal on an input side. When the NF of the optical amplifier has been deteriorated, the OSNR of the light signal input to an optical receiving station turns out to be deteriorated.
In addition, when NF deterioration has become advanced above a certain level, a transmission error has occurred and communication becomes unable. Therefore, it is important to adequately detect the NF deterioration at an early stage.
As a technique of the related art, a technique has been proposed where an NF is obtained on the basis of the inverted population state of amplifying media.
Such a technique is disclosed in Japanese Laid-open Patent Publication No. 8-250785 and Japanese Laid-open Patent Publication No. 6-334238.
As general NF deterioration detection, a method has existed where, using an optical spectrum analyzer, the OSNRs of the input and output light signals of an optical amplifier are directly measured and the NF deterioration is detected.
In this case, an NF is calculated from the OSNR of the spectrum value of the light signal of each of the input stage and the output stage of the optical amplifier, measured in the optical spectrum analyzer, and the calculated NF is compared with an NF initial value, thereby detecting the NF deterioration.
However, since the optical spectrum analyzer is highly expensive, a configuration is not realistic where, for example, the optical spectrum analyzer is provided in each of optical amplifiers included in a system performing multi-stage optical relay and such NF deterioration detection as described above is performed, and hence, it is difficult to apply the configuration to an actual optical communication network.